Turbochargers for gasoline and diesel internal combustion engines are known devices used in the art for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine to spin within the housing. The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor mounted onto an opposite end of the shaft. Thus, rotation of the turbine also causes the air compressor to spin within a compressor housing of the turbocharger that is separate from the exhaust housing. The spinning of the air compressor causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.
The common shaft extending between the turbine and compressor is disposed through a turbocharger center housing that includes a bearing assembly for: (1) facilitating shaft rotation; (2) controlling axially directed shaft thrust effects and radially directed shaft vibrations; and (3) providing necessary lubrication to the rotating shaft to minimize friction effects and related wear. The common shaft as used in turbocharger applications is known to have shaft-rotating speeds on the order of 60,000 to 80,000 rpm. Under such operating conditions it is imperative that the bearing assembly provide sufficient lubrication to the shaft to minimize the extreme friction effects that take place at such high rotating speeds, thereby extending shaft service life.
Bearing assemblies known in the art for turbocharger shaft applications include roller bearings and ball bearings to accommodate the high-speed shaft rotation. However, it has been found that bearing assemblies that make exclusive use of such ball or roller bearings do not provide a desired service life for turbochargers in vehicle applications. Other bearing assemblies known in the art for turbocharger applications make use of sleeve bearings. However, sleeve bearings have been found to be objectionable in such applications because their design do not tolerate a practical degree of shaft imbalance and do not operate to dampen resonant vibrations caused by such imbalance, such imbalance being a characteristic of rotating turbocharger shafts. Further, the inability of such sleeve bearings to accommodate shaft imbalance at such high speeds is known to cause oil film breakdown and metal-to-metal contact, also reducing the shaft operating life.
In an effort to address the disadvantages of these prior art bearing systems, bearing assemblies have been constructed in the form of a free-floating bushing, positioned between the rotating shaft and a stationary housing cavity, that include a roller or ball bearing system. The use of a roller or ball bearing system in conjunction with the free-floating bushing is designed to both provide a desired degree of lubrication to the shaft and to absorb vibration caused by the shaft during rotating movement at such high speeds. Such bearing systems also employ thrust-bearing surfaces to control axial shaft movement during rotary operation.
For example, U.S. Pat. No. 4,641,977 discloses a bearing system comprising an anti-friction rolling bearing that cooperates with a full-floating sleeve to carry the rotating turbocharger shaft. More specifically, the bearing system comprises an outer race having an integral one-piece elongated cylindrical outer bearing surface that is adapted to be carried rotatably on a film of lubricant at its interface with the turbocharger housing. The outer race cooperates with a full-floating sleeve at one of its ends and with the roller bearing at an opposite end. The roller bearing is interposed between the outer race and an inner race that is positioned concentrically around the shaft diameter. An end of the outer race adjacent the rolling bearing includes outwardly projecting surfaces that form thrust bearings stationary machine element. The shaft rotates within the assembly between the roller bearing and the full-floating sleeve.
While the above-discussed bearing system is known to meet the extreme lubrication and damping demands required in turbocharger shaft applications, its design and construction does not lend itself to cost effective production and assembly. Turbochargers for vehicle applications are produced using methods of high-volume manufacturing, thereby requiring that the turbocharger components be constructed in a manner that is consistent with high-efficiency assembly and robotic handling processes.
It is, therefore, desirable that a bearing assembly for use in a turbocharger be constructed in a manner that: (1) meets the lubrication requirements of a rotating turbine shaft under operating conditions; (2) provides necessary damping to a vibrating turbine shaft during operation; (3) provides thrust surfaces to control turbine shaft axial movement during rotary operation; and (4) is constructed in a manner facilitating high-volume manufacturing methods.